1. Field of the Invention
The present invention relates to a work sheet cutting apparatus employed in, for example, a buttonhole sewing machine.
2. Related Art Statement
There is known a work sheet cutting device which includes (a) a cutting member, such as a cutting blade, for cutting a work sheet, such as a work cloth, (b) a support member on which the work sheet is to be received when the cutting member is cutting through the work sheet so as to form an opening or hole in the cloth, and (c) actuating means for displacing the cutting member along a predetermined path between an operative position of the cutting member where the cutting member is received by the support member and a retracted position of the cutting member spaced away from the operative position.
This cutting device is incorporated in, for example, a buttonhole sewing machine, for forming a button hole in a work cloth.
FIGS. 11 and 12 show a known cutting device employed in a buttonhole sewing machine. An actuator bar 103 to the lower end of which a cutting blade 101 is secured is displaced downward by an air cylinder 105 connected to the upper end of the actuator bar 103, so that the blade 101 is applied to a work cloth 107 set on a support member 109 and cooperates with the support 109 to cut through the cloth 107 and thereby form an opening serving as a button hole. A light-shading plate 113 is attached to the actuator bar 103, so that the plate 113 cooperates with a light detector 111 to identify that the cutting blade 101 is located at a predetermined position on its predetermined path between its operative position where the blade 101 is received by the support 109 and its retracted position spaced upward away from the operative position.
Upon operation of a start switch (not shown) provided on the sewing machine, the sewing machine automatically carries out a series of operations, first, forming buttonhole sewing stitches and, subsequently, opening a button hole in the middle of the formed stitches (cf. FIG. 9). FIG. 11 shows the cutting blade 101 or actuator bar 103 positioned at a height or level where the light-shading plate 113 prevents light from being incident to the light detector 111. When, from this position, the blade 101 or bar 103 is displaced downward by the air cylinder 105, the blade 101 will reach its operative position as shown in FIG. 12 where the blade 10i is received by the support 109 and cooperates with the support 109 to cut through the cloth 107. At the operative position of the blade 101, the light-shading plate 113 permits light to be incident to the light detector 111, so that the light detector 111 generates a "DETECTION" signal. The sewing machine identifies, upon generation of the "DETECTION" signal, that the work cloth cutting (or button hole opening) operation has been completed, and therefore supplies a "RETRACT" signal to the air cylinder 105 so as to start displacing the blade 101 upward toward its retracted position.
However, the above described cutting device suffers from some problems.
Since the cutting blade 101 is worn little by little because of use, it is necessary to sharpen or grind the blade 101 by removing it from the actuator bar 103. Thus, the blade 101 becomes shorter little by little. Unless the stoke of displacement of the blade 101 provided by the air cylinder 105 is adjusted or increased, the blade 101 will be unable to reach its operative position on the support 109, so that the blade 109 may fail to form a button hole in the work cloth 107. In the known cutting device, therefore, it is necessary to adjust the position of the light-shading plate 113 or the light detector 111 by a length equal to the shortened length of the blade 101. However, this adjustment is very cumbersome and time-consuming.
For avoiding the above indicated cumbersome adjustment, the stroke of displacement of the blade 101 provided by the air cylinder 105 may be pre-set at a more or less greater value, so that the air cylinder 105 continues to displace the blade 101 downward for a predetermined time duration even after the light detector 111 has generated a "DETECTION" signal. In this case, however, the blade 101 is pressed against the support 109 with excessively large force, which will lead to faster wearing of the blade 101 or even breaking the same 101. Therefore, this measure is not satisfactory.
FIGS. 13 and 14 shows a different known cutting device employed in a known buttonhole sewing machine. This cutting device has a construction basically similar to that of the above described known cutting device.
When a start switch (not shown) is operated on the known sewing machine, the sewing machine automatically carries out a series of operations, first, forming buttonhole sewing stitches, subsequently, opening a button hole in the middle of the formed stitches, and then, cutting a needle thread and a bobbin thread.
FIG. 14 shows a cutting blade 301 or actuator bar 303 located at its operative position where the blade 301 is pressed on a work cloth 307 and is received by a support member 309. If, in this condition, a needle-thread cutting device (not shown) is moved for cutting the needle thread, a pair of scissors of the needle-thread cutter device will however collide with the work sheet cutting device, in particular, cutting blade 301. Therefore, the sewing machine must not cause the needle-thread cutter device to start moving toward the needle thread at the time when the work cloth cutting (or button hole opening) operation has been completed in the condition shown in FIG. 14. Rather, when a light detector 311 detects a light-shading plate 313 and produces a "NON-DETECTION" signal indicating that the blade 301 has been displaced up to its retracted position (i.e., original position or upper limit position) by an air cylinder 305, as shown in FIG. 13, the sewing machine commands the pair of scissors of the needle-thread cutter device to start moving toward the needle thread.
Since this known sewing machine does not start moving the needle-thread cutter device until the cutting blade 301 is retracted to its retracted position, it needs a long cycle time to complete the above indicated series of operations.
However, if the cutting blade 301 has been displaced by a predetermined amount of distance from its operative position up to an intermediate position short of its retracted position, the interference of the blade 301 with other devices can be avoided. It is possible to detect, by adjusting the position of the light-shading plate 313 or light detector 311, that the blade 301 has been displaced to the above indicated "non-interference" position intermediate between the operative and retracted positions of the blade 301. Therefore, the sewing machine may be adapted to start moving the needle-thread cutter device at the time when the blade 301 is displaced up to the "non-interference" position. In this case, the cycle time is shortened. However, another problem remains.
As indicated previously, since the cutting blade 301 is worn little by little during use, it is necessary to sharpen or grind the blade 301 by removing it from the actuator bar 303. Thus, the blade 301 becomes shorter little by little. Unless the stoke of displacement of the blade 301 provided by the air cylinder 305 is adjusted or increased, the blade 301 will be unable to reach its operative position on the support 309, so that the blade 301 may fail to form a button hole in the work cloth 307. In the known cutting device, therefore, it is necessary to increase the stroke of downward displacement of the blade 301 provided by the air cylinder 305, by a length equal to the shortened length of the blade 301. For complying with this stroke adjustment, it will be necessary to adjust the position of the light-shading plate 313 or the light detector 311. Otherwise, the entire cycle time would be increased. However, this adjustment is very cumbersome and time-consuming.